* Introduce a wp_checkdate() function with a single filter to centralise the code that validates dates.
* Improve the error message
* Correctly handle the return value of wp_insert_post which is not always a WP_Error on failure
Fixes#17180
git-svn-id: http://core.svn.wordpress.org/trunk@21922 1a063a9b-81f0-0310-95a4-ce76da25c4cd